Air lubrication system

ABSTRACT

A system for providing an air lubricating layer between a substantially flat bottom ( 3 ) of a vessel ( 1 ) and the water flowing under the bottom as the vessel is moving through the water, whereby the system includes sidewalls ( 5,5′ ) and a top wall ( 4 ) defining a cavity ( 6 ) with an opening ( 13 ) situated in an interface plane that is transversal to the sidewalls ( 5,5′ ), at the level of the flat bottom ( 3 ), the opening having a front end ( 9 ) and a rear end ( 15 ) seen in the length direction of the cavity, an air inlet ( 10 ) spaced from the opening for introducing air into the cavity, whereby the length of the opening ( 13 ) of the cavity ( 6 ) is between 2 and 10 m and the distance (H) of the top wall ( 4 ) from the interface plane is between 0.2 m and 0.5 m.

FIELD OF THE INVENTION

The invention relates to a system for providing an air lubricating layerbetween the hull of a vessel and the water flowing under the hull as thevessel is moving through the water. The invention also relates to adisplacement vessel comprising such a system, to a method of operatingsuch a system in a displacement vessel and to a method of providing sucha system in a displacement vessel.

BACKGROUND OF THE INVENTION

WO 2010/064911 describes how to generate a layer of micro bubbles on thehull of a displacement vessel in order to reduce frictional drag, bymeans of a cavity being provided in a flat bottom area of the hull of avessel, such that an opening of the cavity is at the flat bottom area.Air is injected into the cavity at such a rate that the water level inthe cavity is kept substantially at the level of the hull of the vessel.Due to the forward movement of the vessel, the air in the cavity willmove relative to the water at the speed at which the vessel is moving,or seen from the cavity, the water will flow past the cavity at thatspeed. This difference in velocity between the air and the water causesa so-called Kelvin Helmholtz Instability (KHI) which results in a mixingof air and water at the interface between the water and the air, and ina consequential generation of a layer of small sized air bubbles. Thesmall size of these air bubbles makes them very stable and they tend tostay in the water for a relative long period. The small bubblesgenerated in the cavity are released there from at the rear edge of thecavity such that they form a stable lubricating layer extending a longdistance along the bottom of the hull downstream of the cavity. In thisway energy savings in propulsion of a vessel can be achieved.

It is an object of the invention to provide an improved system forproviding an air lubricating layer between the hull of a vessel and thewater flowing under the hull as the vessel is moving through the water,which can more easily be incorporated in the hull of a vessel.

It is a further object of the present invention to provide an improvedsystem for providing an air lubricating layer that can be operated in anefficient and stable manner in wave conditions.

It is a further object of the invention to provide a displacement vesselincorporating such a system.

It is another object of the invention to provide a method of operatingsuch a system in a displacement vessel.

It is another object of the invention to provide a method atretrofitting such a system into the hull of an existing displacementvessel, without weakening of the existing hull. It is another object toprovide a method of retrofitting without interfering with the cargospace of the vessel.

SUMMARY OF THE INVENTION

To meet these objectives the present invention provides a system forproviding an air lubricating layer between a substantially flat bottomof the hull of a vessel and the water flowing under the bottom as thevessel is moving through the water, whereby the system comprisessidewalls and a top wall defining a cavity with an opening situated inan interface plane that is transversal to the sidewalls, substantiallyat the level of the flat bottom, the opening having a front end and arear end seen in the length direction of the cavity, an air inlet spacedfrom the opening for introducing air into the cavity, wherein the lengthof the opening of the cavity is between 2 m and 10 m, preferably between2 m and 7 m, more preferably between 2 m and 5 m, and the distance (H)of the top wall from the interface plane is between 0.2 m and 0.5 m.

The inventors have realized that a length of the opening of the cavityof between 2 m and 10 m, at a height of 0.2 m to 0.5 m results informing a stable and well-defined lubricating air-water mixture at theinterface between the water and the air in the cavity. This was alsofound for cavities having a length of between 2 m and 5 m, for adistance of the top wall from the interface plane of between 0.2 m and1.5 m, preferably of between 0.2m and 1.0 m, most preferably of between0.2 m and 0.5 m.

Compared to the relatively large opening and large volume of the cavityknown from WO 2010/064911 which is suggested to be between 5 m and 30 mlong, and up to 5 m in height, the inventors have found that the cavitycan be made much smaller while operating in a stable and efficientmanner and at the same time is more easy to incorporate into the hull ofa vessel. In particular when such a system is retrofitted, it isadvantageous that cavity is as small as possible in order to minimizeany weakening effect of the structure of the hull. In particular, acavity height of less than 0.5 m allows the system to be arrangedbetween the two hulls in a double bottom or a double hull vessel, whichmeans that the available cargo space is not affected by theincorporation of the system into the vessel. The cavity known from WO2010/064911 has a height of up to 5 m, which means that it will extendinto the cargo space of the vessel, whereby less cargo can betransported by the vessel. Furthermore, smaller systems require lessmaterial and are easier to handle.

Another disadvantage of a 30 m long cavity, such as known from WO2010/064911, is that waves will start to form inside the cavity, wherebyat the rear end of the cavity such waves will cause an uneven exiting ofthe water-air mixture from the cavity. This means that the lubricatinglayer formed between the hull and the water flowing underneath it willbe uneven and thereby less efficient. The inventor has realized that therelatively short length of the cavity according to the invention and itsrelatively shallow shape are sufficient to form an appropriate water-airmixture at the interface between the water and the air in the cavity,which air layer when exiting the cavity extends across a long length ofthe flat bottom, ensuring a good lubricating effect on the hull of thevessel, and that at the same time no significant wave formation willoccur in such a short cavity. The cavity according to the invention isrelatively insensitive to waves and functions well without loss of aireven when the vessel has large heave, roll and pitch motions.

It has also been found that air consumption by use of a relatively smallcavity can be reduced while still maintaining an effective frictionreducing lubricating effect.

Apart from the more uniform and thereby improved lubricating effectobtained by such a relative short cavity, it is also advantageous thatthe chamber is relative short when it has to be retrofitted into theexisting hull of a vessel. It saves material and labor cost when it hasto be welded into the hull.

Preferably, the cavity has a width between 0.5 m and 1.5 m. Severalcavities can be distributed across the hull of the vessel side by sideand/or behind the other when seen in a length direction.

When at least the portion of the rear end of the cavity being adjacentthe opening is sloping towards the opening, the sloping wall partextending to the position of a substantially horizontal bottom plane,the air-water mixture will be guided smoothly underneath the hull andprovide a uniform lubricating effect even when the vessel is rolling ormoving due to waves on the sea, and thereby ensure an even release ofthe lubricating layer underneath the hull of the vessel. Preferably, thesystem is applied in a flat bottom vessel and the rear end of the cavityslopes downwardly to intersect the bottom plane of the hull in order fora smooth transition of the bubbles formed in the cavity. Preferably, thesloping wall part of the cavity is tangent to the bottom plane at theposition of the bottom plane to provide a stable transition of the airbubble layer from the cavity along the substantially flat bottom of thehull of the vessel, forming a long bubble tail extending over a longdistance in a rearward direction.

Preferably, at least the portion of the rear end of the cavity beingadjacent the opening is convex. This will even further improve themanner in which the lubricating layer is allowed to be introducedunderneath the hull.

Preferably, means are provided to control the volume of air beingintroduced into the cavity proportional to a width of the cavity,dependent upon the speed of the vessel, such that for a cavity of awidth of about 1 m:

at a speed of 4 m/s an airflow of between 7 and 70 m³/h is provided,

at a speed of 5 m/s an airflow of between 15 and 150 m³/h is provided,

at a speed of 6 m/s an airflow of between 25 and 250 m³/h is provided,

at a speed of 7 m/s an airflow of between 45 and 450 m³/h is provided,

at a speed of 8 m/s an airflow of between 70 and 700 m³/h is provided,

at a speed of 9 m/s an airflow of between 15 and 150 m³/h is provided,

at a speed of 10 m/s an airflow of between 100 and 1000 m³/h isprovided,

at a speed of 11 m/s an airflow of between 140 and 1400 m³/h isprovided, and

at a speed of 12 m/s an airflow of between 260 and 2600 m³/h isprovided.

The control system can comprise a processor and memory device connectedto the air compressor feeding the air into the cavity. The inventor hasfound a specific relationship between the speed of the vessel and theamount of air being required in order to provide a given lubricatingeffect, which is about proportional to the width of the cavity. Hence,the airflow can be regulated without the need for any sensors inside thecavity. This greatly simplifies the system itself, and furthermorereduces the cost involved at its installation.

Preferably, the distance between the sidewalls of the cavity decreasesin a forward direction so that the front part of the cavity has a‘dagger’-shape. This shape serves to eliminate or at least reduce anywave formations which might be generated inside the cavity when thewater level is not completely flush with the lower edge of the frontwall.

When the system is formed as a module which, for example, is to beretrofitted into the hull of an existing vessel, it can comprise a flatplate section extending around the opening, said flat plate sectionbeing adapted to be welded into an opening in the hull of a vessel. Thisfacilitates the installing and welding in place of the module in thehull of the vessel.

The invention also provides a system for providing an air lubricatinglayer between a substantially flat bottom of a vessel and the waterflowing under the bottom as the vessel is moving through the water,whereby the system comprises sidewalls and a top wall defining a cavitywith an opening defining an interface plane located at a distance fromthe top wall, substantially at the level of the flat bottom, the openinghaving a front end and a rear end seen, when in use, in the direction offorward movement of the vessel, an air inlet separate from the openingfor introducing air into the cavity, and at least one wave deflectingmember extending inside the cavity in the width direction, at a distancefrom the interface plane, the wave deflecting member being fixed to theside walls and/or top wall and being oriented substantially transverselyto the interface plane.

Such a wave deflecting member facilitates the starting-up of the system.Before the system is activated the cavity will normally be full ofwater. Thus, the system must be able to be activated when the vessel ismoving at its travelling speed through water. When starting to introduceair into the cavity, the water level will be at the top of the cavity,and the flow of water under the hull of the vessel, will cause a veryviolent turbulence inside the cavity, drawing a large volume of air outof the cavity before the level of water has reached the level of theopening. Thus, it requires a very large airflow to start the system,much larger than what is required to run the system at a steady state.Consequently, an air source having a high capacity is required. However,if one or more wave deflecting member(s) are provided inside the cavity,the turbulence generated by the waves emerging from the front end edgeof the cavity, and the turbulence generated there from will be deflectedwhereby less air will be drawn out of the cavity during the initialstartup of the system. Consequently, an air source having a lowercapacity, a capacity which is only slightly larger than what is requiredto run the system at a steady state, is sufficient in order to be ableto start the system.

Preferably the at least one wave deflecting member is curved, whereby itwill even better dampen the wave formation inside the cavity.

The invention also provides a displacement vessel having a waterdisplacement of at least 10.000 tons, preferably 50.000 tons and moreand comprising at least one cavity for providing an air lubricatinglayer between the hull of the vessel and the water flowing under thehull as the vessel is moving through the water, the vessel having asubstantially flat bottom extending substantially across the widththereof, the flat bottom having a length of at least 10 m, preferably atleast 20 m, and a propulsion device for sailing at a speed, the openingof the cavity being substantially at the level of the flat bottom, andsuch that a portion of the flat bottom having a length at least as greatas the length of a cavity, preferable at least 15 m, extends downstreamfrom the, or each, cavity.

In order to create a lubricating layer across the width of the hull of alarge vessel a plurality of cavities can be arranged substantiallyadjacent over at least a part of the width of the hull.

For a long vessel, a plurality of cavities or a plurality of rows ofadjacent transversely extending cavities can be arranged after eachother in the longitudinal direction of the hull. The lubricating effectof the bubbles emitted from a cavity will begin to become less after 50to 100 m, whereby, in a large vessel which can be up to 400 m long, itis appropriate to provide at least 4-8 cavities distributed in thelength direction, or 4-8 rows of adjacent transversely extendingcavities, after each other in the longitudinal direction of the vessel.

The invention also provides a method of operating a system for providingan air lubricating layer between the hull of a vessel having asubstantially flat bottom and the water flowing under the hull as thevessel is moving through the water, the method comprising:

-   -   introducing air into the cavity to expel water out of the        cavity,    -   continue introducing air into the cavity at such a rate that the        water level in the cavity will be kept substantially at the        level of the outer surface of the flat bottom, whereby when the        vessel is moving forwards through water the air in the cavity        will mix with the water at the air-water interface, thereby        forming a air-water mixture, and    -   allowing said mixture to exit said cavity below the rear edge        thereof, along the width of the opening, such that it will form        a lubricating layer along the flat bottom downstream of the        cavity.

Preferably the air is injected into the cavity in such a way as to avoiddirect impingement onto the air-water mixture. In order for the KHI towork properly, and to avoid the formation of large bubbles which willhave only a poor lubricating effect, which does not last very long, itis important that the air-water interface is not affected by a strongjet of injected air. Preferable the air is injected into the chamber viaa large opening, whereby the speed of the air at the inlet opening canbe kept low, preferably below 5-10 m/s.

The invention also provides a method of providing a system as set outabove, in the hull of a vessel, the method comprising:

-   -   cutting a hole in the hull of the vessel,    -   positioning the opening such that it communicates with the hole        and welding the walls of the cavity to the hull of the vessel,    -   connecting an air duct to the cavity.

According to this method, the system can easily be provided in the hullof an existing vessel, whereby energy saving for propulsion of up to 15%can be achieved in existing vessels.

BRIEF DESCRIPTION OF THE DRAWINGS

Some embodiments of a system for providing an air lubricating layerbetween the hull of a vessel and the water flowing under the hull as thevessel is moving through the water according to the present inventionwill by way of example be described in detail with reference to theaccompanying drawings. In the drawings:

FIG. 1 shows a schematic side view of a vessel which as an examplecomprises two cavities arranged after each other in the hull of thevessel,

FIG. 2 shows a bottom view of the vessel,

FIG. 3 shows a schematic side view of system according to the inventionhaving curved wave deflecting members extending transversely in thecavity,

FIG. 4 shows a schematic side view of system according to the inventionhaving substantially plane wave deflecting members extendingtransversely in the cavity,

FIG. 5 shows a schematic view of the opening of a cavity, seen from thebottom of the vessel.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a schematic side of a vessel 1 according to the inventionhaving a hull 2 and a bottom surface 3. The hull 2 has a length Lh offor instance between 50 m and 400 m, measured along the bottom surface3.

As shown in FIG. 2 the bottom surface 3 of the vessel 1 comprises tworows of cavities 6, each having five cavities 6.1 a to 6.1 e and 6.2 ato 6.2 e arranged next to each another across the width Wv of thevessel. The width Wv may be between 10 and 50 m, for instance between 15m and 20 m. The rows are shown as having a general V-shape, but thecavities could also be arranged on a straight of curved line, or in anyother pattern. The number of cavities is selected according to the widthWv of the bottom surface 3. It is, however, also possible to have asingle cavity in the bottom surface 3.

The cavities preferable all have the same dimensions, but they couldalso have different dimension, for example the cavities near the side ofthe vessel could have a different shape and/or size.

FIGS. 3-5 shows a cavity 6 having a front end 9, a rear end 15, a topwall 4, a rear wall 16, two side walls 5, 5′, and an air inlet 10, anair supply duct 11 connected to the air inlet 10. The cavity 6 furthercomprises an opening 13 which is substantially flush with bottom surface3 of the hull. The opening 13 has a length Lc which is relatively shortcompared the vessel length Lh and which lies between 2 m and 10 m. Thewidth W of the opening 13 (see FIG. 5) is preferable between 0.5 m and1.5 m. The cavity has a height H, measured from the bottom surface 3 toa top wall 4, which may be between 0.2 m and for instance 0.5 m.

Other small size cavities having a length Lc between 1.5 m and 5 m at aheight H of between 0.2 m and 1.5 m, preferably 0.2 m and 1 m alsoprovide an efficient and stable air lubricating layer.

Near a front end 9 of the cavity 6, an air inlet 10 is provided, whichis connected to an air supply duct 11. A compressor 12 takes inatmospheric air through a duct 11 and supplies compressed air to thecavity 6 in order to expel water from the cavity. A controller 20, suchas a computer device, is connected to the compressor 12 for operatingthe compressor depending on the speed of the vessel.

When the vessel is sailing through the water the moving water across thewater-air interface in the cavity 6 results in a Kelvin HelmholtzInstability and forms small-sized bubbles 14. These bubbles escape via abubble outflow region at the rear end 15 of the cavity 6. At the rearend 15, the cavity 6 has a downwardly sloping surface, forming awedge-shaped space near the rear 15 of the cavity. From this outflowregion, the bubbles 14 spread towards the aft 18 of the hull 2, to covera majority of the bottom surface 3. The downwardly sloping surface canbe formed as straight wall 16, or as a curved wall 16. The preferredembodiment of FIG. 3 shows a curved rear wall 16 which at the position15 of the bottom surface 3 is tangent with said bottom surface 3. Inorder for the bubbles to be guided smoothly out of the cavity it issufficient that the lower part of the rear wall 16 is curved orinclined. For a smooth transition of the bubbles from the cavity isimportant the rear wall 16 extends all the way to the position 15 of thebottom surface 3. It is not necessary that the portion of the rear wall16 being adjacent the top wall 4 is curved or inclined. This portion ofthe rear wall 16 could, for example, be vertical.

As shown in FIGS. 3 and 4, a number of wave deflecting members 7, 7′, 7″extends transversely inside the cavity 6. These wave deflecting membersstay clear from the top wall 4 and/or side walls 5, 5′for allowing afree flow of air through the cavity. Alternatively, the wave deflectingmembers extend to the top wall 4 and/or side walls 5,5 but aretransparent for air, for instance by being provided with perforations,or forming a mesh like pattern. These wave deflecting members 7, 7′, 7″facilitate the starting-up of the system. Before the system is activatedthe cavity 6 will normally be full of water. The wave deflecting members7, 7′, 7″ serve to deflect the waves emerging from the front end 9 ofthe cavity 6 when the vessel in moving forward through the water, andthe turbulence generated there from will be deflected whereby less airwill be drawn out of the cavity 6 during the initial startup of thesystem. A single wave deflecting member 7 might be sufficient in someinstances, whereas an improved effect is obtained by providing more wavedeflecting members, such as 3 or 5, or more than 5. The wave deflectingmember(s) can be fixed to the sidewalls 5, 5′ and/or to the top wall 4.

The at least one wave deflecting member 7 can be curved, or it can beformed by a substantially plane element arranged vertically, or at aninclined angle, such that it sloped towards the aft 18 of the hull 2 ina downward direction. The members 7, 7′, 7″ can be solid or, optionally,be provided with holes or openings extending from a front to a rear sideof a member 7, 7′, 7″.

1-13. (canceled)
 14. A system for providing an air lubricating layerbetween a substantially flat bottom (3) of the hull (2) of adisplacement vessel (1) and the water flowing under the bottom as thevessel is moving through the water, whereby the system comprisessidewalls (5,5′) and a top wall (4) defining a cavity (6) with anopening (13) situated in an interface plane that is transversal to thesidewalls (5,5′), at the level of the flat bottom (3), the openinghaving a front end (9) and a rear end (15) seen in the length directionof the cavity, an air inlet (10) spaced from the opening for introducingair into the cavity, whereby the length of the opening (13) of thecavity (6) is between 2 and 10 m, preferably between 2 and 7 m, morepreferably between 2 and 5 m, and the distance (H) of the top wall (4)from the interface plane is between 0.2 m and 0.5 m, wherein the cavity(6) has a width (W) of between 0.5 m and 1.5 m, at least a portion (16)of the rear end (15) of the cavity (6) sloping from the top wall (4) tothe interface plane when going in a rearward direction, the sloping wallpart extending to the position of a substantially horizontal bottomplane to intersect the bottom plane.
 15. A system for providing an airlubricating layer between a substantially flat bottom (3) of the hull(2) of a displacement vessel (1) and the water flowing under the bottomas the vessel is moving through the water, whereby the system comprisessidewalls (5,5′) and a top wall (4) defining a cavity (6) with anopening (13) situated in an interface plane that is transversal to thesidewalls (5,5′), at the level of the flat bottom (3), the openinghaving a front end (9) and a rear end (15) seen in the length directionof the cavity, an air inlet (10) spaced from the opening for introducingair into the cavity, whereby the length of the opening (13) of thecavity (6) is between 2 m and 5 m, and the distance (H) of the top wall(4) from the interface plane is between 0.2 m and 1.5 m, preferablybetween 0.2 m and 1.0 m, most preferably between 0.2 m and 0.5 m,wherein the cavity (6) has a width (W) of between 0.5 m and 1.5 m, atleast a portion (16) of the rear end (15) of the cavity (6) sloping fromthe top wall (4) to the interface plane when going in a rearwarddirection, the sloping wall part extending to the position of asubstantially horizontal bottom plane to intersect the bottom plane. 16.A system according to claim 14, the sloping wall part being tangent tothe bottom plane at the position of the bottom plane.
 17. A systemaccording to claim 14, whereby at least the portion (16) of the rear end(15) of the cavity (6) being adjacent the opening (13) is convex.
 18. Asystem according to claim 14, comprising means (12) to control thevolume of air being introduced into the cavity (6) proportional to awidth (W) of the cavity, dependent upon the speed of the vessel, suchthat for a cavity of a width of about 1 m: at a speed of 4 m/s anairflow of between 7 and 70 m³/h is provided, at a speed of 5 m/s anairflow of between 15 and 150 m³/h is provided, at a speed of 6 m/s anairflow of between 25 and 250 m³/h is provided, at a speed of 7 m/s anairflow of between 45 and 450 m³/h is provided, at a speed of 8 m/s anairflow of between 70 and 700 m³/h is provided, at a speed of 9 m/s anairflow of between 15 and 150 m³/h is provided, at a speed of 10 m/s anairflow of between 100 and 1000 m³/h is provided, at a speed of 11 m/san airflow of between 140 and 1400 m³/h is provided, and at a speed of12 m/s an airflow of between 260 and 2600 m³/h is provided.
 19. A systemaccording to claim 14, a distance between the sidewalls (5, 5′)decreasing when going towards the front (9).
 20. A system according toclaim 14, comprising a flat plate section (8) extending around theopening (13), said flat plate section (8) being adapted to be weldedinto an opening in the hull (2) of a vessel to be flush with the hull.21. A displacement vessel (1) having a water displacement of at least10,000 tons, preferably at least 50,000 tons and comprising a number ofsystems according to claim 14 for providing an air lubricating layerbetween the hull (2) of the vessel and the water flowing under the hullas the vessel is moving through the water, the vessel having asubstantially flat bottom (3), the flat bottom (3) having a length (Lh)of at least 10 m, preferably at least 20 m, and a propulsion device forsailing at a speed, the interface plane of the cavity (6) beingsubstantially at the level of the flat bottom (3), and such that aportion of the flat bottom having a length at least as great as thelength (Lc) of a cavity, preferable at least 15 m, extends downstreamfrom the at least one cavity (6), the vessel comprising a plurality ofcavities (6.1 a-6.1 e and 6.2 a-6.2 e) distributed in the longitudinaldirection of the flat bottom (3).
 22. Displacement vessel according toclaim 21, the flat bottom extending substantially across the width (Wv)of the vessel.
 23. A vessel according to claim 21, comprising aplurality of cavities (6.1 a-6.1 e and 6.2 a-6.2 e) arranged side byside over at least a part of the width of the flat bottom (3).
 24. Amethod of operating the system according to claim 14 in a displacementvessel (1) having a water displacement of at least 10,000 tons,preferably at least 50,000 tons and comprising at least one system forproviding an air lubricating layer between the hull (2) of the vesseland the water flowing under the hull as the vessel is moving through thewater, the vessel having a substantially flat bottom (3), the flatbottom (3) having a length (Lh) of at least 10 m, preferably at least 20m, and a propulsion device for sailing at a speed, the interface planeof the cavity (6) being substantially at the level of the flat bottom(3), and such that a portion of the flat bottom having a length at leastas great as the length (Lc) of a cavity, preferably at least 15 m,extends downstream from the at least one cavity (6), the methodcomprising: introducing air into the cavity to expel water out of thecavity, continue introducing air into the cavity at such a rate that thewater level in the cavity will be kept substantially at the level of theouter surface of the flat bottom, whereby, when the vessel is movingforwards through water the air in the cavity will mix with the water atthe air-water interface, thereby forming a air-water mixture, andallowing said mixture to exit said cavity below the rear edge thereof,along the width of the opening (13) where the sloping wall partintersects the bottom plane, such that it will form a lubricating layeralong the flat bottom downstream of the cavity.
 25. A method accordingto claim 24, wherein the air is injected into the cavity in such a wayas to avoid direct impingement onto the air-water interface.
 26. Amethod of providing a system according to claim 20 in a displacementvessel (1) having a water displacement of at least 10,000 tons, forproviding an air lubricating layer between the hull (2) of the vesseland the water flowing under the hull as the vessel is moving through thewater, the vessel having a substantially flat bottom (3), the methodcomprising: cutting a hole in the hull of the vessel welding the flatplate section (8) into the hole in the hull (2) of a vessel to be flushwith the hull, and connecting an air duct to the cavity.
 27. A systemaccording to claim 15, the sloping wall part being tangent to the bottomplane at the position of the bottom plane.
 28. A system according toclaim 15, whereby at least the portion (16) of the rear end (15) of thecavity (6) being adjacent the opening (13) is convex.
 29. A systemaccording to claim 15, comprising means (12) to control the volume ofair being introduced into the cavity (6) proportional to a width (W) ofthe cavity, dependent upon the speed of the vessel, such that for acavity of a width of about 1 m: at a speed of 4 m/s an airflow ofbetween 7 and 70 m³/h is provided, at a speed of 5 m/s an airflow ofbetween 15 and 150 m³/h is provided, at a speed of 6 m/s an airflow ofbetween 25 and 250 m³/h is provided, at a speed of 7 m/s an airflow ofbetween 45 and 450 m³/h is provided, at a speed of 8 m/s an airflow ofbetween 70 and 700 m³/h is provided, at a speed of 9 m/s an airflow ofbetween 15 and 150 m³/h is provided, at a speed of 10 m/s an airflow ofbetween 100 and 1000 m³/h is provided, at a speed of 11 m/s an airflowof between 140 and 1400 m³/h is provided, and at a speed of 12 m/s anairflow of between 260 and 2600 m³/h is provided.